Dominant Myopathy Research Articles

G.O.7 Distal myopathy with upper limb predominance caused by filamin C haploinsufficiency

Abstract Recently three different FLNC mutations were related to a peculiar phenotype of dominant distal myopathy with a distinct involvement of the hand muscles. Objective: We report the detailed clinical findings and underlying genetic defect in the Bulgarian family. Methods: We performed neurological, electrophysiological, radiological and histopathological analyses of 13 patients and thirteen at-risk but asymptomatic individuals from three generations. Genome-wide parametric linkage analysis was followed by bidirectional sequencing of the filamin C (FLNC) gene. We characterized the identified nonsense mutation at cDNA and protein level. Results: Based on clinical findings, no known myopathy subtype was implicated in our distal myopathy patients. Light microscopic analysis of affected muscle tissue showed no specific hallmarks, however the electron microscopy revealed changes compatible with myofibrillar myopathy. Linkage studies delineated a 9.76 Mb region on chromosome 7q22.1-q35 containing filamin C (FLNC), a gene previously associated with myofibrillar myopathy. Mutation analysis revealed a novel c.5160delC frameshift deletion in all patients. The mutation results in a premature stop codon (p.Phe1720LeufsX63) that triggers nonsense-mediated mRNA decay. FLNC transcript levels were reduced in muscle and lymphoblast cells from affected subjects and partial loss of FLNC in muscle tissue was confirmed by protein analysis. Conclusions: The FLNC mutation that we identified is distinct in terms of the associated phenotype, muscle morphology and underlying molecular mechanism, thus extending the currently recognized clinical and genetic spectrum of filaminopathies. We conclude that filamin C is a dosage-sensitive gene and that FLNC haploinsufficiency can cause a specific type of myopathy in humans.

C.P.14 A four-generation Japanese family with autosomal dominant nemaline myopathy associated with dilated cardiomyopathy

Nemaline myopathy (NEM) is the most common nondystrophic congenital mopathy. NEM is a clinically and genetically heterogeneous myopathy characterized by the presence of nemaline bodies in skeletal muscle. Cardiac muscle involvement has been shown to be rare. Seven causative genes of NEM have been identified: ACTA1, TPM3, TPM2, NEB, TNNT1, KBTBD13 and CFL2. To study clinical characteristics and molecular genetics of a Japanese family of an autosomal dominant NEM associated with dilated cardiomyopathy (DCM). This four-generation Japanese family showed progressive proximal muscular atrophy and DCM with dominant inheritance. The type 1 predominance and nemaline rods were shown in skeletal muscles of the proband and his father. Genomic DNA was extracted from five individuals (two affected, two non-affected and one married-in). Genotyping was carried out using Affymetrix SNP Array 6.0 and parametric linkage analysis was performed using SNP HiTLink (BMC Bioinformatics 2009;10:121). Candidate genes were analyzed by the Sanger method. Results: The linkagae analysis revealed the total size of candidate region was approximately 800Mb. By nucleotide sequence analysis, mutations were not identified in all exons of the genes for autosomal dominant NEM: ACTA1, TPM3 and TPM2. Parametric linkage analysis excluded the loci of the remaining all known NEM genes: NEB, TNNT1, KBTBD13 and CFL2. The causative genes for dominant DCM (LMNA, LDB3, TNNT2, TTN, DES, EYA4, CSRP3, TCAP, ABCC9, PLN, ACTC1, MYH7, TMPO, PSEN1, TPM1, ACTN2, NEXN, RBM20, MYH6, TNNI3 and BAG3) were also excluded by the results of nucleotide sequence analysis or linkage analysis respectively. The molecular genetics study using linkage and sequencing analyses revealed the present family with dominant NEM not linked to the known seven causative genes for hereditary NEM. The present study suggested a novel causative gene for NEM with DCM.

G.P.39 DNAJB6 myopathy in Japanese cohort

DNAJB6 myopathy is an autosomal dominant myopathy characterized by middle-aged onset of lower limb muscle weakness and presence of rimmed-vacuoles (RVs) and cytoplasmic inclusions (CIs) on muscle pathology. DNAJB6 is a member of DNAJ (HSP40) family, having roles in anti-protein aggregation and growth inhibition of cardiomyocytes. So far, only three different mutations have been identified in 11 unrelated families. To identify and characterize DNAJB6 myopathy in Japanese cohort. Patients and methods: We searched for DNAJB6 mutations by direct sequencing in 103 LGMD families and 89 families with RVs and/or CIs on muscle pathology. Clinical and pathological characteristics were analyzed. We found four families with DNAJB6 myopathy. One family had a novel p.F93I substitution in the G/F domain and three had the same p.F93L mutation as previously reported. The patient carrying p.F93I substitution was a 36-year-old male and manifested slowly progressive lower limb weakness from 30s. No cardiac or brain involvement was seen. His father and elder brother showed similar clinical features. Serum CK levels was elevated to 1044IU/L and EMG showed myogenic pattern. Patients carrying the p.F93L mutation also showed adult onset mild myopathy. Muscle images commonly showed preferential involvement of posterior lower limb muscles with preserved rectus femoris (RF) and tibialis anterior (TA) muscles. Skeletal muscles showed myopathic changes with a few fibers with RVs and small cytoplasmic/nuclear inclusions. On immunohistochemistry, accumulations of DNAJB6, TDP43 and the other interacted proteins were seen. Four Japanese families with DNAJB6 myopathy were identified including one with a novel mutation. All patients showed similar clinical and pathological findings. Preservation of RF and TA was characteristic. The G/F domain of DNAJB6 may have important roles in skeletal muscle.

Myosinopathies: pathology and mechanisms

The myosin heavy chain (MyHC) is the molecular motor of muscle and forms the backbone of the sarcomere thick filaments. Different MyHC isoforms are of importance for the physiological properties of different muscle fiber types. Hereditary myosin myopathies have emerged as an important group of diseases with variable clinical and morphological expression depending on the mutated isoform and type and location of the mutation. Dominant mutations in developmental MyHC isoform genes (MYH3 and MYH8) are associated with distal arthrogryposis syndromes. Dominant or recessive mutations affecting the type IIa MyHC (MYH2) are associated with early-onset myopathies with variable muscle weakness and ophthalmoplegia as a consistent finding. Myopathies with scapuloperoneal, distal or limb-girdle muscle weakness including entities, such as myosin storage myopathy and Laing distal myopathy are the result of usually dominant mutations in the gene for slow/β cardiac MyHC (MYH7). Protein aggregation is part of the features in some of these myopathies. In myosin storage myopathy protein aggregates are formed by accumulation of myosin beneath the sarcolemma and between myofibrils. In vitro studies on the effects of different mutations associated with myosin storage myopathy and Laing distal myopathy indicate altered biochemical and biophysical properties of the light meromyosin, which is essential for thick filament assembly. Protein aggregates in the form of tubulofilamentous inclusions in association with vacuolated muscle fibers are present at late stage of dominant myosin IIa myopathy and sometimes in Laing distal myopathy. These protein aggregates exhibit features indicating defective degradation of misfolded proteins. In addition to protein aggregation and muscle fiber degeneration some of the myosin mutations cause functional impairment of the molecular motor adding to the pathogenesis of myosinopathies.

A Novel Autosomal Dominant Inclusion Body Myopathy Linked to 7q22.1-31.1

We describe a novel autosomal dominant hereditary inclusion body myopathy (HIBM) that clinically mimics limb girdle muscular dystrophy in a Chinese family. We performed a detailed clinical assessment of 36 individuals spanning four generations. The age of onset ranged from the 30s to the 50s. Hip girdle, neck flexion and axial muscle weakness were involved at an early stage. This disease progressed slowly, and a shoulder girdle weakness appeared later in the disease course. Muscle biopsies showed necrotic, regenerating, and rimmed vacuolated fibers as well as congophilic inclusions in some of the fibers. Electron micrograph revealed cytoplasmic inclusions of 15–21 nm filaments. A genomewide scan and haplotype analyses were performed using an Illumina Linkage-12 DNA Analysis Kit (average spacing 0.58 cM), which traced the disease to a new locus on chromosome 7q22.1–31.1 with a maximum multi-point LOD score of 3.65. The critical locus for this unique disorder, which is currently referred to as hereditary inclusion body myopathy 4 (HIBM4), spans 8.78 Mb and contains 65 genes. This localization raises the possibility that one of the genes clustered within this region may be involved in this disorder.

Clinicopathological features of centronuclear myopathy in Japanese populations harboring mutations in dynamin 2

BackgroundMissense mutations in dynamin 2 gene (DNM2) are associated with autosomal dominant centronuclear myopathy (CNM) with characteristic histopathological findings of centrally located myonuclei in a large number of muscle fibers. MethodsTo identify Japanese CNM caused by DNM2 mutations (DNM2-CNM), we sequenced DNM2 in 22 unrelated Japanese patients who were pathologically diagnosed with CNM. The clinical and pathological findings of DNM2-CNM in patients were reviewed. ResultsWe identified 3 different heterozygous missense mutations (p.E368K, p.R369W, and p.R465W) in 4 probands from 4 families. Clinically, calf muscle atrophy and pes cavus are features that are highly suggestive of DNM2-CNM among all CNMs. Pathologically, all 4 DNM2-CNM patients showed a radial distribution of myofibrils in scattered fibers, type 1 fiber atrophy, type 1 fiber predominance, and type 2C fibers. None of the non-DNM2-CNM patients exhibited all the 4 abovementioned pathological features, although some patients showed radial distribution without type 1 fiber atrophy and/or type 2C fibers. DiscussionThese results indicate that the clinicopathological features of DNM2-CNM are rather homogeneous and can be distinguished from the features of non-DNM2-CNM.

Titin mutation segregates with hereditary myopathy with early respiratory failure

In 2001, we described an autosomal dominant myopathy characterized by neuromuscular ventilatory failure in ambulant patients. Here we describe the underlying genetic basis for the disorder, and we define the neuromuscular, respiratory and radiological phenotype in a study of 31 mutation carriers followed for up to 31 years. A combination of genome-wide linkage and whole exome sequencing revealed the likely causal genetic variant in the titin (TTN) gene (g.274375T>C; p.Cys30071Arg) within a shared haplotype of 2.93 Mbp on chromosome 2. This segregated with the phenotype in 21 individuals from the original family, nine subjects in a second family with the same highly selective pattern of muscle involvement on magnetic resonance imaging and a third familial case with a similar phenotype. Comparing the mutation carriers revealed novel features not apparent in our original report. The clinical presentation included predominant distal, proximal or respiratory muscle weakness. The age of onset was highly variable, from early adulthood, and including a mild phenotype in advanced age. Muscle weakness was earlier onset and more severe in the lower extremities in nearly all patients. Seven patients also had axial muscle weakness. Respiratory function studies demonstrated a gradual deterioration over time, reflecting the progressive nature of this condition. Cardiomyopathy was not present in any of our patients despite up to 31 years of follow-up. Magnetic resonance muscle imaging was performed in 21 affected patients and revealed characteristic abnormalities with semitendinosus involvement in 20 of 21 patients studied, including 3 patients who were presymptomatic. Diagnostic muscle histopathology most frequently revealed eosinophilic inclusions (inclusion bodies) and rimmed vacuoles, but was non-specific in a minority of patients. These findings have important clinical implications. This disease should be considered in patients with adult-onset proximal or distal myopathy and early respiratory failure, even in the presence of non-specific muscle pathology. Muscle magnetic resonance imaging findings are characteristic and should be considered as an initial investigation, and if positive should prompt screening for mutations in TTN. With 363 exons, screening TTN presented a major challenge until recently. However, whole exome sequencing provides a reliable cost-effective approach, providing the gene of interest is adequately captured.

Mutations in the Nuclear Encoded Novel Mitochondrial Protein CHCHD10 Cause an Autosomal Dominant Mitochondrial Myopathy (IN7-2.002)

Objective: To report a novel causative gene for an autosomal dominant mitochondrial myopathy. Background Mitochondrial myopathies belong to a larger group of systemic diseases caused by morphological or biochemical abnormalities of mitochondria. Mitochondrial disorders can be caused by mutation either in the mitochondrial or in the nuclear genome. Design/Methods: Patients are part of previously reported Hispanic kindred with autosomal dominant mitochondrial myopathy (Heiman-Patterson et al 1997). Linkage analysis was established using fluorescent microsatellite markers and illumina genome wide SNP markers. Candidate gene was identified using MitoCarta (Vamsi K. Mootha, personal communication). Candidate gene was sequenced using Beckman Coulter CEQ 8000 DNA analysis system. The cDNA of wild type, single and double mutant gene were cloned in pCMV-Myc vector and co-transfected with pDsRed2-mito in HEK293 cells. The co-localization was monitored with confocal microscopy. Results: In this five generation Hispanic family we established linkage to chromosome 22q11. Using the tools of integrative gneomics, we established CHCHD10/ C22orf16 as the high scoring mitochondrial candidate gene in our minimal candidate region. Sequence analysis revealed a double heterozygote missense mutation in this gene. These two mutations segregated with disease phenotype and were absent in 500 Caucasians and Hispanic non affected controls. Both wild types, single and double mutant proteins co-localized with mitochondria. Conclusions: We identified a new nuclear encoded mitochondrial gene ( CHCHD10 ) as the cause of an autosomal dominant mitochondrial myopathy. CHCHD10 encodes a non characterized 142 aa protein with a target mitochondrial signaling peptide and a conserved CHCH domain. This CHCH domain is present in many intermembrane space proteins, such as the family of small Tim proteins. The exact function of this protein remains to be determined. Our findings expands the spectrum of mitochondrial myopathies caused by nuclear gene dysfunction. Disclosure: Dr. Ajroud-Driss has nothing to disclose. Dr. Fecto has nothing to disclose. Dr. Ajroud has nothing to disclose. Dr. Siddique has nothing to disclose.

Focal Presentations of Facioscapulohumeral Dystrophy (P04.082)

Objective: To identify additional focal presentations of FSHD. Background Facioscapulohumeral dystrophy (FSHD) is an autosomal dominant myopathy, classically presenting with facial and shoulder girdle weakness. Recently, monomelic lower limb presentations of genetically confirmed FSHD have been reported. Design/Methods: A retrospective review of our academic center medical record database of all FSHD cases seen from 1996 to 2011. Of 139 cases identified, 8 had DNA confirmation with a focal presentation. Demographic and clinical data was abstracted. Results: There were 8 cases (4 male) with mean age at onset 39 years (range 18-63) and age at diagnosis 45 years (range 20-74). Presenting symptoms were monomelic lower limb atrophy (3); unilateral shoulder atrophy (3); and axial weakness (2). Examination showed isolated focal weakness in five. CK was normal or mildly elevated. All had a single family member with “weakness”. Co-existent unusual symptoms included dyspnea (2), S1 radicular pain with calf atrophy (2), peripheral neuropathy (1) and post-traumatic onset (1). EMG showed myopathic changes in all, apart from one case with severe denervation later followed by myopathic changes. DNA analysis showed D4Z4 EcoRI fragment size ranging from 20 to 37 kilobases. Conclusions: The clinical phenotype of FSHD should be expanded to include focal atrophy with myopathic EMG findings; axial weakness; or dyspnea at presentation. These cases are challenging due to lack of typical FSHD signs and coexistent symptoms suggestive of an alternative diagnosis. Disclosure: Dr. Hassan has nothing to disclose. Dr. Jones has nothing to disclose. Dr. Milone has nothing to disclose. Dr. Kumar has nothing to disclose.

Mutation spectrum in the large GTPase dynamin 2, and genotype-phenotype correlation in autosomal dominant centronuclear myopathy

Centronuclear myopathy (CNM) is a genetically heterogeneous disorder associated with general skeletal muscle weakness, type I fiber predominance and atrophy, and abnormally centralized nuclei. Autosomal dominant CNM is due to mutations in the large GTPase dynamin 2 (DNM2), a mechanochemical enzyme regulating cytoskeleton and membrane trafficking in cells. To date, 40 families with CNM-related DNM2 mutations have been described, and here we report 60 additional families encompassing a broad genotypic and phenotypic spectrum. In total, 18 different mutations are reported in 100 families and our cohort harbors nine known and four new mutations, including the first splice-site mutation. Genotype-phenotype correlation hypotheses are drawn from the published and new data, and allow an efficient screening strategy for molecular diagnosis. In addition to CNM, dissimilar DNM2 mutations are associated with Charcot-Marie-Tooth (CMT) peripheral neuropathy (CMTD1B and CMT2M), suggesting a tissue-specific impact of the mutations. In this study, we discuss the possible clinical overlap of CNM and CMT, and the biological significance of the respective mutations based on the known functions of dynamin 2 and its protein structure. Defects in membrane trafficking due to DNM2 mutations potentially represent a common pathological mechanism in CNM and CMT.

A Centronuclear Myopathy – Dynamin 2 Mutation Impairs Autophagy in Mice

Dynamin 2 (Dnm2) is involved in endocytosis and intracellular membrane trafficking through its function in vesicle formation from distinct membrane compartments. Heterozygous (HTZ) mutations in the DNM2 gene cause dominant centronuclear myopathy or Charcot-Marie-Tooth neuropathy. We generated a knock-in Dnm2R465W mouse model expressing the most frequent human mutation and recently reported that HTZ mice progressively developed a myopathy. We investigated here the cause of neonatal lethality occurring in homozygous (HMZ) mice. We show that HMZ mice present at birth with a reduced body weight, hypoglycemia, increased liver glycogen content and hepatomegaly, in agreement with a defect in neonatal autophagy. In vitro studies performed in HMZ embryonic fibroblasts point out to a decrease in the autophagy flux prior to degradation at the autolysosome. We show that starved HMZ cells have a higher number of immature autophagy-related structures probably due to a defect of acidification. Our results highlight the role of Dnm2 in the cross talk between endosomal and autophagic pathways and evidence a new role of Dnm2-dependent membrane trafficking in autophagy which may be relevant in DNM2-related human diseases.

Dynamin 2 mutations in Charcot–Marie–Tooth neuropathy highlight the importance of clathrin-mediated endocytosis in myelination

Mutations in dynamin 2 (DNM2) lead to dominant intermediate Charcot-Marie-Tooth neuropathy type B, while a different set of DNM2 mutations cause autosomal dominant centronuclear myopathy. In this study, we aimed to elucidate the disease mechanisms in dominant intermediate Charcot-Marie-Tooth neuropathy type B and to find explanations for the tissue-specific defects that are associated with different DNM2 mutations in dominant intermediate Charcot-Marie-Tooth neuropathy type B versus autosomal dominant centronuclear myopathy. We used tissue derived from Dnm2-deficient mice to establish an appropriate peripheral nerve model and found that dominant intermediate Charcot-Marie-Tooth neuropathy type B-associated dynamin 2 mutants, but not autosomal dominant centronuclear myopathy mutants, impaired myelination. In contrast to autosomal dominant centronuclear myopathy mutants, Schwann cells and neurons from the peripheral nervous system expressing dominant intermediate Charcot-Marie-Tooth neuropathy mutants showed defects in clathrin-mediated endocytosis. We demonstrate that, as a consequence, protein surface levels are altered in Schwann cells. Furthermore, we discovered that myelination is strictly dependent on Dnm2 and clathrin-mediated endocytosis function. Thus, we propose that altered endocytosis is a major contributing factor to the disease mechanisms in dominant intermediate Charcot-Marie-Tooth neuropathy type B.

Autosomal dominant myofibrillar myopathy with arrhythmogenic right ventricular cardiomyopathy 7 is caused by a DES mutation

Using exome sequencing we searched for the genetic cause of autosomal dominant myofibrillar myopathy with arrhythmogenic right ventricular cardiomyopathy (ARVC) in a Swedish family. A heterozygous C-to-T transition, c.1255C>T, p.Pro419Ser in the desmin gene on chromosome 2q35, was identified. Previous studies had demonstrated linkage to chromosome 10q22.3, but no causative mutation had been found in that region. Sanger sequencing of DNA from 17 family members confirmed the heterozygous c.1255C>T desmin mutation in seven out of ten family members that had been classified as affected in the previous study. Our new results demonstrate the usefulness of next-generation sequencing, and the diagnostic difficulties with some forms of dominantly inherited muscle diseases as they can display a wide clinical and morphological variability even within a given family.

Exome sequencing reveals DNAJB6 mutations in dominantly‐inherited myopathy

To identify the causative gene in an autosomal dominant limb-girdle muscular dystrophy (LGMD) with skeletal muscle vacuoles. Exome sequencing was used to identify candidate mutations in the studied pedigree. Genome-wide linkage was then used to narrow the list of candidates to a single disease-associated mutation. Additional pedigrees with dominant or sporadic myopathy were screened for mutations in the same gene (DNAJB6) using exome sequencing. Skeletal muscle from affected patients was evaluated with histochemistry and immunohistochemical stains for dystrophy-related proteins, SMI-31, TDP43, and DNAJB6. Exome analysis in 3 affected individuals from a family with dominant LGMD and vacuolar pathology identified novel candidate mutations in 22 genes. Linkage analysis excluded all variants except a Phe93Leu mutation in the G/F domain of the DNAJB6 gene, which resides within the LGMD locus at 7q36. Analysis of exome sequencing data from other pedigrees with dominant myopathy identified a second G/F domain mutation (Pro96Arg) in DNAJB6. Affected muscle showed mild dystrophic changes, vacuoles, and abnormal aggregation of proteins, including TDP-43 and DNAJB6 itself. Mutations within the G/F domain of DNAJB6 are a novel cause of dominantly-inherited myopathy. DNAJB6 is a member of the HSP40/DNAJ family of molecular co-chaperones tasked with protecting client proteins from irreversible aggregation during protein synthesis or during times of cellular stress. The abnormal accumulation of several proteins in patient muscle, including DNAJB6 itself, suggest that DNAJB6 function is compromised by the identified G/F domain mutations.

Upper Thoracic Myelopathy Caused by Delayed Neck Extensor Weakness in Myotonic Dystrophy

Myotonic dystrophy is the most common autosomal dominant myopathy in adults. Our patient, a 41 year-old female suffering from myotonic muscular dystrophy, developed upper thoracic myelopathy due to hypertrophy of the ligamentum flavum and the posterior longitudinal ligament. She had a typical hatchet face and ptosis with "head hanging forward" appearance caused by neck weakness. Motor weakness, sensory changes and severe pain below T4 level, along with urinary incontinence began 3 months ago. Genetic and electrodiagnostic studies revealed myotonic dystrophy type 1. Magnetic resonance imaging of the spine showed loss of cervical lordosis and spinal cord compression due to hypertrophied ligamentum flavum and posterior longitudinal ligament at T1 to T3 level. We concluded that her upper thoracic myelopathy was likely related to the thickness of the ligamentum flavum and posterior longitudinal ligament due to repetitive mechanical stress on her neck caused by neck muscle weakness with myotonic dystrophy.

Effects of Pathogenic Proline Mutations on Myosin Assembly

Laing distal myopathy (MPD1) is a genetically dominant myopathy characterized by early and selective weakness of the distal muscles. Mutations in the MYH7 gene encoding for the β-myosin heavy chain are the underlying genetic cause of MPD1. However, their pathogenic mechanisms are currently unknown. Here, we measure the biological effects of the R1500P and L1706P MPD1 mutations in different cellular systems. We show that, while the two mutations inhibit myosin self-assembly in non-muscle cells, they do not prevent incorporation of the mutant myosin into sarcomeres. Nevertheless, we find that the L1706P mutation affects proper antiparallel myosin association by accumulating in the bare zone of the sarcomere. Furthermore, bimolecular fluorescence complementation assay shows that the α-helix containing the R1500P mutation folds into homodimeric (mutant/mutant) and heterodimeric [mutant/wild type (WT)] myosin molecules that are competent for sarcomere incorporation. Both mutations also form aggregates consisting of cytoplasmic vacuoles surrounding paracrystalline arrays and amorphous rod-like inclusions that sequester WT myosin. Myosin aggregates were also detected in transgenic nematodes expressing the R1500P mutation. By showing that the two MPD1 mutations can have dominant effects on distinct components of the contractile apparatus, our data provide the first insights into the pathogenesis of the disease.

Phenotypic Variability in a Large Czech Family with a Dynamin 2–Associated Charcot-Marie-Tooth Neuropathy

: Mutations in the Dynamin 2 gene (DNM2) cause autosomal dominant centronuclear myopathy or autosomal dominant (AD) Charcot-Marie-Tooth (CMT) disease. Here the authors report one large Czech family with 15 members affected with an AD CMT phenotype of extraordinary variability. Genetic linkage analysis using SNP arrays revealed a locus of about 9.6 Mb on chromosome 19p13.1–13.2. In this critical interval, 373 genes were located. The only gene herein known to be associated with an intermediate type of CMT was Dynamin 2 (DNM2). Subsequent sequence analysis of the DNM2 gene in the index patient revealed a novel missense mutation p.Met580Thr. This missense mutation segregated with the neuropathy, indicating the causal character of this mutation. The phenotype of CMT in this family shows mild to moderate impairment with relatively preserved upper limbs and a very broad range of the onset of clinical symptoms from an early onset around the age of 12 to the late onset during the fifth decade. Electrophysiology showed an intermediate type of peripheral neuropathy. The motor median nerve conduction velocity varied from 36 m/s to normal values with signs of asymmetrical affection of peripheral nerves. No additional symptoms such as cranial nerve involvement, cataract, and signs of neutropenia or myopathy syndrome were observed in any member of the family yet. The progression was slow with no loss of ambulation. The authors suggest that the characterization of clinical variability in a single family may help to direct the genetic analysis directly to the rarely observed DNM2 mutations.

RNA Interference Improves Myopathic Phenotypes in Mice Over-expressing FSHD Region Gene 1 (FRG1)

Muscular dystrophies, and other diseases of muscle, arise from recessive and dominant gene mutations. Gene replacement strategies may be beneficial for the former, while gene silencing approaches may provide treatment for the latter. In the last two decades, muscle-directed gene therapies were primarily focused on treating recessive disorders. This disparity at least partly arose because feasible mechanisms to silence dominant disease genes lagged behind gene replacement strategies. With the discovery of RNA interference (RNAi) and its subsequent development as a promising new gene silencing tool, the landscape has changed. In this study, our objective was to demonstrate proof-of-principle for RNAi therapy of a dominant myopathy in vivo. We tested the potential of adeno-associated viral (AAV)-delivered therapeutic microRNAs, targeting the human Facioscapulohumeral muscular dystrophy (FSHD) region gene 1 (FRG1), to correct myopathic features in mice expressing toxic levels of human FRG1 (FRG1(-high) mice). We found that FRG1 gene silencing improved muscle mass, strength, and histopathological abnormalities associated with muscular dystrophy in FRG1(-high) mice, thereby demonstrating therapeutic promise for treatment of dominantly inherited myopathies using RNAi. This approach potentially applies to as many as 29 different gene mutations responsible for myopathies inherited as dominant disorders.

Distinct distal myopathy phenotype caused by VCP gene mutation in a Finnish family

Inclusion body myopathy with Paget disease and frontotemporal dementia (IBMPFD) is caused by mutations in the valosin-containing protein (VCP) gene. We report a new distal phenotype caused by VCP gene mutation in a Finnish family with nine affected members in three generations. Patients had onset of distal leg muscle weakness and atrophy in the anterior compartment muscles after age 35, which caused a foot drop at age 50. None of the siblings had scapular winging, proximal myopathy, cardiomyopathy or respiratory problems during long-term follow-up. Three distal myopathy patients developed rapidly progressive dementia, became bedridden and died of cachexia and pneumonia and VCP gene mutation P137L (c.410C>T) was then identified in the family. Late onset autosomal dominant distal myopathy with rimmed vacuolar muscle pathology was not sufficient for exact diagnosis in this family until late-occurring dementia provided the clue for molecular diagnosis. VCP needs to be considered in the differential diagnostic work-up in patients with distal myopathy phenotype.

Mutations in the N-terminal Actin-Binding Domain of Filamin C Cause a Distal Myopathy

Linkage analysis of the dominant distal myopathy we previously identified in a large Australian family demonstrated one significant linkage region located on chromosome 7 and encompassing 18.6 Mbp and 151 genes. The strongest candidate gene was FLNC because filamin C, the encoded protein, is muscle-specific and associated with myofibrillar myopathy. Sequencing of FLNC cDNA identified a c.752T>C (p.Met251Thr) mutation in the N-terminal actin-binding domain (ABD); this mutation segregated with the disease and was absent in 200 controls. We identified an Italian family with the same phenotype and found a c.577G>A (p.Ala193Thr) filamin C ABD mutation that segregated with the disease. Filamin C ABD mutations have not been described, although filamin A and filamin B ABD mutations cause multiple musculoskeletal disorders. The distal myopathy phenotype and muscle pathology in the two families differ from myofibrillar myopathies caused by filamin C rod and dimerization domain mutations because of the distinct involvement of hand muscles and lack of pathological protein aggregation. Thus, like the position of FLNA and B mutations, the position of the FLNC mutation determines disease phenotype. The two filamin C ABD mutations increase actin-binding affinity in a manner similar to filamin A and filamin B ABD mutations. Cell-culture expression of the c.752T>C (p.Met251)Thr mutant filamin C ABD demonstrated reduced nuclear localization as did mutant filamin A and filamin B ABDs. Expression of both filamin C ABD mutants as full-length proteins induced increased aggregation of filamin. We conclude filamin C ABD mutations cause a recognizable distal myopathy, most likely through increased actin affinity, similar to the pathological mechanism of filamin A and filamin B ABD mutations.